This is an incomplete playbook concept. And some random associated thoughts.
Inspired by: events in our current game, Sam Worthington's half-terminator character in the trailers for Terminator: Salvation (never seen the movie), and every other thing in which a main character is empowered by some crazy inhuman force that they have to struggled to keep in check.
The character starts with high Weird, but no extra Weird moves, having to buy them later on or from other playbooks.
THE NATURE OF THE BEAST
This is somewhat like the Faceless's mask traits or the Driver's car, in that you select certain options from the list below (these are just some example ideas).
When the character gives into the beast within, they can roll +Weird to Seize By Force or any other Weird-based move they've purchased (otherwise, they can't roll these moves), but:
* The Monster Revealed: on a hit, they must choose to frighten, even if that's their only choice. If they are not Seizing By Force, you frighten in addition to whatever other results you achieve.
* Playing with Fire: on a 7-9, the GM gets Hold+1, on a failure, Hold 3. Spend this hold to have the character's affliction interfere with any roll.
* Collateral Damage: on a 7-9, name which bystander also takes harm (as established); on a failure, the bystander takes harm (as established) instead of your target.
OTHER OPTIONS:
* The Anguish: Weird can never be highlighted. Instead, when the character intentionally chooses to use merely their human characteristics, keeping the beast repressed, and fails or chooses not to act... mark experience.
* The Growing Hunger: Each time you roll Weird, including opening your brain, mark a box under this move, each time you reach 5; mark a new option for the Nature of the Beast. It awakens.
OTHER THOUGHTS
It might also be interesting to make this a playbook that is meant to be applied later on in the game, one to be switched to and not a complete starting playbook in its own right. Basically, you'd switch to it if you discovered, in play, that your character had become something else monstrous or was secretly something insane the whole time.